Chapter 27 - Stress-degree-day concept and crop water stress index

Abstract

This chapter describes the stress-degree-day and the crop water stress index (CWSI) to evaluate water stress in plants. They were developed in the 1970s by Ray Jackson and Sherwood Idso at the US Department of Agriculture's Water Conservation Laboratory in Phoenix, Arizona. The procedures to determine them are based on measurements of canopy temperature obtained with handheld infrared thermometers. The CWSI is being used widely to determine water stress in crops. An example is given to show how to calculate the CWSI. One needs to measure the difference between canopy temperature and air temperature and plot those measurements against vapor pressure deficit. The importance of having a wide range in vapor pressure deficit values is pointed out. The normalized difference vegetation index, which measures reflectance from plant leaves in the red and near-infrared regions, is defined and distinguished from the CWSI, which is based on temperature measurements. An appendix gives the biography of Idso.